1. Field of the Invention
The present invention relates to a bimorph type piezoelectric head actuator comprising piezoelectric bodies and an elastic shim material for use as a head actuator for automatic tracking in a video cassette recorder (VCR), for example.
2. Description of the Prior Art
Demand for high performance head actuators for faithfully recording and reproducing video signals to a magnetic tape medium has grown in conjunction with the trends toward high functionality and image quality in VCRs and other video recording and reproducing apparatuses.
A conventional magnetic head apparatus of this type is described below with reference to the figures.
FIG. 14a is a partially cut-away perspective view of the conventional magnetic head apparatus according to Japanese patent laid-open Publication No. 55-139630 wherein the head actuator 500 is provided on a head drum 501.
In the flying head helical scan VCR in which the head actuator shown in FIG. 14a is applied, the magnetic tape 502 is wound around the head drum 501 at a predetermined angle, and recording tracks T (solid lines, FIG. 14b) are formed on the magnetic tape 502 at an angle corresponding to the tape speed and the velocity of the flying heads. When playing back the magnetic signal from the magnetic tape at a playback speed differing from the recording speed, as during still, slow motion, high speed, or reverse playback, the scanning path of the magnetic heads differs from the recording track. As a result, the magnetic heads scan a path offset from the recording tracks as shown by the dotted line in FIG. 14b, resulting in guard band noise, crosstalk, and other problems.
Flying head VCRs of this type therefore feature a tracking head actuator 500 enabling the magnetic heads to accurately trace the recording tracks in each of the variable speed playback modes. A typical head actuator uses a bimorph plate as shown in FIG. 15.
The structure and operating principle of this head actuator is described below with reference to FIG. 15. This bimorph plate 601 is mounted on the head base 602 and comprises two piezoelectric ceramic bodies 603a and 603b sandwiching a metal reinforcing plate 604 that also functions as a middle electrode.
The bimorph plate 601 comprising the head actuator is a cantilever structure as shown in FIG. 15 supported on just one end by the head base 602 or other solid member with the other end connected to the magnetic head 605. This bimorph plate 601 is manufactured by laminating the two piezoelectric ceramic bodies 603a and 603b through the metal reinforcing plate 604 in the fixed-end side 601B of the bimorph plate 601, and directly bonding the two piezoelectric ceramic bodies 603a and 603b together with an adhesive on the magnetic head 605 side 601A.
With the bimorph plate 601 thus constructed, the bimorph plate 601 is flexibly displaced in direction C (FIG. 15) by the electric field generated by a voltage applied between the electrodes 606 and 607 on the outside surface of the piezoelectric ceramic bodies of the part 601B including the metal reinforcing plate 604, and drives part 601A to displace in the same direction by the electric field generated by a voltage applied between the electrodes 608 and 609 provided on the piezoelectric ceramic bodies.
The bimorph plate 601 is thus flexibly displaced by the electrodes 606 and 607 to change the position of the magnetic head 605 relative to the magnetic tape 610, and the bimorph plate 601 is extended in the direction of the magnetic tape 610 by electrodes 608 and 609 to maintain a constant gap between the magnetic tape 610 and magnetic head 605 (specifically, a constant projection of the magnetic head 605) and prevent a loss of the recording/playback signal due to magnetic head 605 tracking by stabilizing the head-tape contact pressure.
The problem of recording/playback signal deterioration caused by the relative angle of the magnetic head 605 to the magnetic tape 610 is not, however, resolved.
The head actuator shown in Japanese patent application number 57-60528 was proposed to resolve this problem. This head actuator is described below with reference to FIG. 16.
This head actuator comprises first, second, third, and fourth bimorph plates 701,702, 704 and 705, two electro-mechanical conversion elements 703 and 706, a head support 707, and a magnetic head 708. In the first electro-mechanical conversion element 703, the first bimorph plates 701 are sandwiched between the two second bimorph plates 702 at the fixed end; in the second electro-mechanical conversion element 706, the third bimorph plates 704 are sandwiched between the two fourth bimorph plates 705 at the fixed end. The head support 707 has a U-shaped cross section connecting the free ends of the first and second electro-mechanical conversion element. The magnetic head 708 is attached to the head support 707.
In this head actuator, an electric field is applied to the piezoelectric ceramic bodies of the first to fourth bimorph plates to expand the piezoelectric ceramic bodies on side Sa and contract them on side Sb. To displace the actuator in the opposite directions, the applied electric fields are reversed. The first and second electro-mechanical conversion elements can thus be displaced in the directions of arrow C in FIG. 16 by controlling the direction of the applied electric field.
In general, the relationship between the displacement .xi. (equation (1)) of the DC field and the resonance frequency f (equation (2)) of the bimorph plates in a one dimensional model in which a metal reinforcing plate ("elastic shim" hereinafter) is sandwiched between piezoelectric bodies can be expressed by equations (1) and (2) below. ##EQU1## where d.sub.31 is the piezoelectric constant, D is the piezoelectric body length, t is the piezoelectric body thickness, t.sub.1 is the elastic shim thickness, .rho. is the density of the piezoelectric body, .rho..sub.1 is the density of the elastic shim, S.sub.11 is the elastic modulus of the piezoelectric body, S.sub.1 is the elastic modulus of the elastic shim, and k.sub.31 is the coupling factor. From equations (1) and (2) above we know that the resonance frequency and displacement are inversely related as increasing one decreases the other.
In the head actuator shown in FIG. 16, however, the displacement obtained by the first and third bimorph plates 701 and 704 is increased by the second and fourth bimorph plates 702 and 705, and greater displacement can be achieved without lowering the resonance frequency of the electromechanical conversion element.
Furthermore, the spacing angle between the magnetic head 708 and magnetic tape 709 can be set to essentially zero, and deterioration of the recording/playback signal can be greatly improved, because the magnetic head 708 can be moved parallel to the magnetic tape 709 by means of the head support 707 connecting the first and second electro-mechanical conversion elements.
The problem with this conventional head actuator, however, is that it is not possible to simultaneously improve the displacement and resonance frequency.
In addition, because the piezoelectric bodies forming the bimorph element are fastened directly to the base or mounting frame, stress is concentrated in the fixed part of the piezoelectric bodies when the bimorph elements are driven, resulting in deteriorated performance due to cracking and other factors.
In addition, the position of the magnetic head relative to the magnetic tape changes with time because the bimorph element is deformed, irrespective of the applied strain, due to plastic deformation of the fixed end of the piezoelectric ceramic caused by the weight of the bimorph element, the mass of the magnetic head, and other factors. Detecting the position of the magnetic head with a strain gauge or other device is not possible, and precise position control is difficult.
In addition, it is difficult to uniformly bond the piezoelectric ceramic to the base, and wide variations in the resonance frequency and displacement can result due the bonding state and conditions. This makes it difficult to achieve stable characteristics.